Bad Bug Book:
Foodborne Pathogenic Microorganisms and Natural Toxins Handbook
Factors Affecting the Growth of Microorganisms in Foods
Food is a chemically complex matrix, and predicting whether, or how fast, microorganisms will grow in any given food is difficult. Most foods contain sufficient nutrients to support microbial growth. Several factors encourage, prevent, or limit the growth of microorganisms in foods, the most important are aw, pH, and temperature.
aw: (Water Activity or Water Availability). Water molecules are loosely oriented in pure liquid water and can easily rearrange. When other substances (solutes) are added to water, water molecules orient themselves on the surface of the solute and the properties of the solution change dramatically. The microbial cell must compete with solute molecules for free water molecules. Except for Staphylococcus aureus, bacteria are rather poor competitors, whereas molds are excellent competitors.
aw varies very little with temperature over the range of temperatures that support microbial growth. A solution of pure water has an aw of 1.00. The addition of solute decreases the aw to less than 1.00.
|Percent NaCl (w/v)||Molal||Water Activity (aw)|
The aw of a solution may dramatically affect the ability of heat to kill a bacterium at a given temperature. For example, a population of Salmonella typhimurium is reduced tenfold in 0.18 minutes at 60°C if the aw of the suspending medium is 0.995. If the aw is lowered to 0.94, 4.3 min are required at 60°C to cause the same tenfold reduction.
An aw value stated for a bacterium is generally the minimum aw which supports growth. At the minimum aw, growth is usually minimal, increasing as the aw increases. At aw values below the minimum for growth, bacteria do not necessarily die, although some proportion of the population does die. The bacteria may remain dormant, but infectious. Most importantly, aw is only one factor, and the other factors (e.g., pH, temperature) of the food must be considered. It is the interplay between factors that ultimately determines if a bacterium will grow or not. The aw of a food may not be a fixed value; it may change over time, or may vary considerably between similar foods from different sources.
pH: (hydrogen ion concentration, relative acidity or alkalinity). The pH range of a microorganism is defined by a minimum value (at the acidic end of the scale) and a maximum value (at the basic end of the scale). There is a pH optimum for each microorganism at which growth is maximal. Moving away from the pH optimum in either direction slows microbial growth.
A range of pH values is presented here, as the pH of foods, even those of a similar type, varies considerably. Shifts in pH of a food with time may reflect microbial activity, and foods that are poorly buffered (i.e., do not resist changes in pH), such as vegetables, may shift pH values considerably. For meats, the pH of muscle from a rested animal may differ from that of a fatigued animal.
A food may start with a pH which precludes bacterial growth, but as a result of the metabolism of other microbes (yeasts or molds), pH shifts may occur and permit bacterial growth.
Temperature. Temperature values for microbial growth, like pH values, have a minimum and maximum range with an optimum temperature for maximal growth. The rate of growth at extremes of temperature determines the classification of an organism (e.g., psychrotroph, thermotroph). The optimum growth temperature determines its classification as a thermophile, mesophile, or psychrophile.
INTERPLAY OF FACTORS AFFECTING MICROBIAL GROWTH IN FOODS: Although each of the major factors listed above plays an important role, the interplay between the factors ultimately determines whether a microorganism will grow in a given food. Often, the results of such interplay are unpredictable, as poorly understood synergism or antagonism may occur. Advantage is taken of this interplay with regard to preventing the outgrowth of C. botulinum. Food with a pH of 5.0 (within the range for C. botulinum) and an aw of 0.935 (above the minimum for C. botulinum) may not support the growth of this bacterium. Certain processed cheese spreads take advantage of this fact and are therefore shelf stable at room temperature even though each individual factor would permit the outgrowth of C. botulinum.
Therefore, predictions about whether or not a particular microorganism will grow in a food can, in general, only be made through experimentation. Also, many microorganisms do not need to multiply in food to cause disease.